Because the log-periodic dipole antenna ("LPDA") affords a theoretical infinite bandwidth, LPDAs are invariably proposed when broadband antenna performance is demanded. In practice, the frequency range within which an LPDA is able to operate is limited by the detail of the feed point and the length of the largest dipole, respectively. For a conventional LPDA, the length of the largest dipole is on the order of one half the wavelength of the lowest operating frequency. This physical requirement precludes the use of LPDAs in some circumstances.
The conventional LPDA is defined primarily by two design parameters: Alpha, the enclosed angle, and Tau, the ratio of the distance between adjacent dipoles. Alpha controls the length of the antenna structure, and Tau determines the number of dipole elements. LPDAs with Alpha narrower than 15.degree. and Tau greater than 0.9 generally provide high gain and directivity as well as nearly frequency-independent performance. In addition, for each Alpha there exists a correlatively optimal value of Tau. Deviation from the optimal value tends to result in a degradation in antenna performance. In practice, antenna designs seek to minimize Tau because reduced dipole spacing requires less material and results in shorter assembly time. However, with Alpha less than 15.degree., the LPDA will tolerate a relatively large range of Tau without significant performance degradation. For this reason, most size-reduction experiments have been conducted using LPDAs with relatively small Alpha.
A number of design techniques for LPDAs with small Alphas have been demonstrated. An example is the reduced-size antenna described in U.S. Pat. No. 3,543,277 to Pullara, entitled "Reduced Size Broadband Antenna." An antenna disclosed therein is characterized by an Alpha of 12.degree. and a Tau of 0.95.
Various other efforts have been directed to the reduction of the size of the LPDAs. (See, for example, Stephenson, "Log-Periodic Helical Dipole Array," WESCON Digest (1963); E. Young, "Foreshortened Log-Periodic Dipole Array," WESCON Digest (1963); Defonzo, "Reduced Size Log-Periodic Antennas," Microwave Journal (December, 1972)). Many resulting techniques were directed to capacitive "T" or "U" loading, or to replacing the linear dipoles with helical dipoles. However, such techniques have been able to achieve a reduction in the width of the LPDAs only at the expense of increased antenna boomlength. This is due to the fact that these types of dipoles exhibit higher Q than conventional dipoles. Consequently, an additional number of "foreshortened dipoles" need be added to the LPDA structure in order to preserve the LPDA's frequency-independent, or broadband, characteristics. In addition, these techniques tend to increase the design complexity of the LPDA, primarily because foreshortening requires more than the straightforward replacement of linear dipoles of an existing LPDA with reconfigured, foreshortened dipoles. As a result, the design of the foreshortened LPDA has historically involved a large number of "cut and try" processes.
An improved technique subsequently discovered by the inventor of the instant invention and disclosed in U.S. Pat. No. 3,732,572 (hereinafter "'572") allows simple replacement, on a one-to-one basis, of the linear dipoles of a conventional LPDA with foreshortened counterparts. For further explication, see Kuo, "Size-Reduced Log-Periodic Dipole Array Antenna," Microwave Journal (December, 1972). This technique circumvents the experimental approach to foreshortened LPDA design. (The information contained in the '572 patent and the technical article authored by the inventor of the as provided in Section 608.01(p) of the Manual of Patent Examining Procedure.)
The theoretical principle supporting the invention disclosed in '572 derives from the electromagnetic analogy that may be drawn between the rectangular waveguide and the slot antenna. As is well known, the cutoff wavelength of the fundamental mode of a rectangular waveguide is twice the width of the waveguide. Furthermore, the cutoff frequency of a ridged waveguide is known to be lower than that of a rectangular waveguide of identical width and height. Because the resonant frequency of a slot antenna is the analog of the waveguide resonant frequency, the antenna resonant frequency may be expected to correspond to the waveguide cutoff frequency. Specifically, the resonant frequency of a slot antenna may be expected to be reduced when its interior profile is formed in the fashion of the cross section of a ridged waveguide. Finally, because a dipole antenna is an analog, as defined by Babinets' principle, of the slot antenna, it is expected that the physical length of the dipole is susceptible of foreshortening when formed in the shape of a ridged waveguide. Empirical investigation has justified the above hypotheses. To wit: The invention embodied in '572 has permitted the physical size of a conventional dipole antenna to be foreshortened by as much as 35 to 40 percent, without significant effect on the antenna's electrical characteristics. Foreshortening is accomplished by imparting to the dipole the interior cross-sectional profile of a ridged rectangular waveguide. However, even with access to the above technique, foreshortening of antennas with Alphas in excess of 45.degree. is difficult to obtain. Heretofore, no practitioner is known to have successfully reduced the width of LPDAs with Alpha greater than or equal to 45.degree. at frequency higher than VHF range, 300 MHz.
The difficulty in foreshortening LPDAs with Alphas about 45.degree. lies with the conventional LPDA itself. As a result, LPDAs with Alpha greater than 45.degree. simply are not commercially available for microwave frequency range. To date, there has been only limited investigation of the performances and anomalies of LPDAs with large Alpha. (See, for example, Bantin, C. and Balmain, K. "Study of Compressed Log-Period Dipole Antennas," IEEE Transaction on Antennas and Propagation (March 1970)). The incentive to develop an LPDA with large Alpha becomes apparent when it is understood that the boomlength of an LPDA with an Alpha of 45.degree. is approximately one fifth the boomlength of an LPDA with Alpha of 12.degree.. Thus, while numerous efforts have been undertaken to reduce the width of the LPDAs with relatively small Alpha, very little effort has been devoted to the investigation of "short" LPDAs. In fact, conventional LPDAs with large Alpha fail to retain their frequency independence unless special techniques are brought to bear.
When the Alpha of an LPDA is increased, the optimized value of Tau is normally reduced in order to maintain proper spacing between the adjacent dipoles. By doing so, the number of near-resonant dipoles is reduced in proportion to the reduction in Tau. When the number of near-resonant dipoles in the active region is insufficient to radiate a substantial portion of the excitation currents, the residue currents will propagate and excite the 1.5L or, perhaps, the 2.5L dipoles. Radiation from these larger dipoles results in deterioration of the frequency-independent characteristics of the LPDAs.
One method which will prevent the larger dipoles from radiating is to increase the feedline characteristic impedance by increasing the spacing of the two-wire balanced feedline. This approach forces a greater proportion of the energy from the feedline into the near-resonant dipoles and therefore reduces the magnitude of the residue currents. As a result, the LPDA typically assumes a mean input impedance of 140 ohms or greater. A broadband impedance transformer is then required to transform the input impedance to 50 ohms. This is very difficult to accomplish at microwave frequencies, especially when the maximum operating frequency approaches 20 GHz.
Another method involves the replacement of the linear dipoles with radiators with lower Q. The triangularly shaped dipole is such a radiator. Its Q decreases as the base of the triangularly shaped dipole increases. Of course, when the base dimension approaches zero, a linear dipole is obtained. These lower Q radiators will couple an enhanced proportion of energy from the feedline, with an effect identical to that obtained by introducing additional radiators into the active region. LDPAs with Alpha equal to 45.degree. have been built and tested, and no anomalies were observed. These results indicate that the largest proportion of the excitation currents are radiated by the near 0.5L dipoles.
A disadvantage of the triangularly shaped dipole is that it resonates at frequencies greater than the resonant frequency of a linear dipole of the same length. For a triangularly shaped dipole that has a height-to-base ratio of 5:1, wherein "height" is defined as one-half of the dipole length, the triangular dipole must be approximately 20% longer than a linear dipole that resonants at the same frequency. Thus, an LPDA which has such triangularly shaped dipoles must be 20% wider and longer than an LPDA with linear dipoles operating over the same frequency range. Clearly this is to be avoided, inasmuch as the salient purpose of the triangularly shaped dipole is to reduce the size of the antenna structure.
Consequently, what is desired is a heretofore unavailable LPDA configuration for antennas with Alpha approaching 45.degree.. The desired LPDA configuration should be amendable to "foreshortening" techniques such as that disclosed in '572. An optimal configuration will circumvent the deterioration in broadband performance attendant heretofore known techniques. Preferably the chosen technique will eliminate the need for a broadband impedance transformer such as is invoked by approaches involving increased spacing of the balanced feedline. Specifically, to the extent triangular radiating elements are employed, it will be necessary to devise an approach that mitigates the additional length that the triangular radiator must assume in order to resonate at the same frequencies as the linear dipole equivalent.
The subject invention is implemented, in one form, by an antenna comprising a coaxial feedline that includes a first coaxial portion and a second coaxial portion, the antenna elements being disposed, in a predetermined fashion, along the lengths of the respective coaxial portions. The first and the second coaxial portions are juxtapositioned so as to exhibit an axial separation that increases in a direction along the length of the coaxial portions. The characteristic impedance of the feedline concomitantly increases along that direction. Antenna elements are disposed along the feedline so that elements of relatively low Q are disposed at positions of relatively low characteristic impedance. Conversely, elements of relatively higher Q are disposed at positions of relatively higher characteristic impedance. More specifically, the antenna consists of two complementary sections with elements disposed in alternately opposite directions from the first and the second coaxial portions.